1. Field of the Invention
This invention relates to an improved flame retardant honeycomb comprising a flame retardant thermoplastic binder having a melt point of from 120° C. to 350° C. and a limiting oxygen index of 26 or greater, methods for making the honeycomb, and articles comprising the honeycomb. In a preferred embodiment the paper in the honeycomb has a flame classification of UL-94 V-0.
2. Description of Related Art
Paper-based honeycomb is typically formed by (1) applying adhesive resin to sheets of paper along predetermined lines, called node lines, (2) adhering several sheets of paper along these node lines to form a stack, with the node lines of each sheet offset to the adjacent sheets, (3) expanding the stack to form a honeycomb having defined cell walls, (4) impregnating the cell walls of the honeycomb with structural resin by submerging the honeycomb in a liquid resin, and (5) curing the resin with heat. U.S. Pat. No. 5,137,768 to Lin; U.S. Pat. No. 5,789,059 to Nomoto; and U.S. Pat. No. 6,544,622 to Nomoto; disclose honeycombs made from sheets made from high modulus para-aramid materials. These honeycombs are highly prized for structural applications due to their high stiffness and high strength to weight ratio. Generally these honeycombs are made from papers comprising para-aramid fibers, pulp, and/or other fibrous materials plus a binder.
The tendency of a paper to extinguish or spread a flame once ignited can be quantified using Underwriters Laboratories, Inc. test method UL 94, “Standard for Flammability of Plastic Materials for Parts in Devices and Appliances”. This testing protocol is used to distinguish a paper's burning characteristics under controlled laboratory conditions. Six of the classifications resulting from this testing relate to materials commonly used in manufacturing enclosures, structural parts, and insulators found in consumer electronic products (5VA, 5 VB, V-0, V-1, V-2, HB). For the purposes of paper used in structural honeycomb, the highest performance, and a desirable objective for flame retardant paper is to pass the V-0 flammability rating.
U.S. Pat. No. 4,557,961 discloses a light-weight, fire-retardant structural panel comprising a honeycomb core formed of aluminum, paper treated to improve its fire resistance characteristics, or the like. This treatment is accomplished by preferably dipping the combustible core material in a liquid bath of phenolic resin and/or a copolymer of vinylidene fluoride and hexafluoropropene so as to render the otherwise combustible paper fire retardant.
In some enduses, honeycomb made from papers containing high modulus fibers and a thermoplastic binder are desired, however, many thermoplastics are very flammable. It is also very costly to post-treat a honeycomb after it has been manufactured. Therefore what is needed is a method to make a flame retardant honeycomb by using a paper containing a flame retardant thermoplastic binder.